The invention relates to a method of increasing heat transfer efficiency by using an improved heat exchanger comprising at least one duct defining a fluid flow path, each duct having an inlet, an outlet and at least two steps protruding into the hollow core of the duct, with the steps being configured so that one step is smaller in dimension than the next step by stepwise and that within the hollow core, they are arranged in an order of increasing dimension toward the outlet from the inlet so as to form a hollow channel having a stepwise decreasing inner dimension.